Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of inkjets, which are arranged in one or more printheads, onto an image receiving surface. In a direct inkjet printer, the printheads eject ink drops directly onto the surface of a print medium such as a paper sheet or a continuous paper web. In an indirect inkjet printer, the printheads eject ink drops onto the surface of an intermediate image receiving member such as a rotating imaging drum or belt. During printing, the printheads and the image receiving surface move relative to one other and the inkjets eject ink drops at appropriate times to form an ink image on the image receiving surface. A controller in the printer generates electrical signals, also known as firing signals, at predetermined times to activate individual inkjets in the printer. The ink ejected from the inkjets can be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, some inkjet printers use phase change inks that are loaded in a solid form and delivered to a melting device. The melting device heats and melts the solid phase change ink to a liquid form that is supplied to a printhead for printing as liquid drops onto the image receiving surface.
During operation, some inkjets in one or more printheads fail to operate due to contaminants that clog nozzles or due to other malfunctions in the printhead. As used herein, the term “inoperable inkjet” refers to an inkjet that fails to eject ink drops onto the predetermined locations of an image receiving surface in a reliable manner during a printing operation. Inoperable inkjets may fail to eject ink drops entirely, eject drops only intermittently, or eject drops onto incorrect locations on the image receiving surface.
Existing compensation methods for inoperable inkjets identify pixel locations in binary halftoned image data that correspond to inoperable inkjets and redistribute the “orphan” pixels from the inoperable inkjet to neighboring inkjets to reduce the perceptible impact of the inoperable inkjet. However, the hardware and software implementations of prior art printers are not suited for use in printers that eject ink drops of different sizes from two or more arrays of inkjets in a print zone. As used herein, the term “multi-level” as used to apply to a printer or to image data for a printed image refers to configurations in which a combination of multiple drop sizes form printed images. For example, in a printer that forms images using two different drop sizes, each halftoned pixel has a total of four potential values or “levels” (e.g. no drops, one small drop, one large drop, or both a small and large drop) instead of the traditional binary image data that only includes two values for drop/no drop. Consequently, improvements systems and methods for inoperable inkjet compensation methods in multi-level printers would be beneficial.